1. Field of the Invention
The invention pertains to the field of semiconductor process chambers, and to methods of reducing contaminants in such chambers and in wafers processed in such chambers.
2. Description of the Related Art
As semiconductor device densities increase and line widths decrease, the control of impurities and contaminants is increasingly important. The presence of impurities during the fabrication process can cause decreased yields, non-uniform device performance characteristics and can increase leakage currents, among other ill effects.
It is thus crucial to insure the purity of semiconductor layers during their formation. One method of creating layers on semiconductor wafers is by Chemical Vapor Deposition, or CVD. CVD utilizes a controlled chemical reaction in a gas or in the vapor phase to deposit thin films on a solid surface, such as a semiconductor substrate. Insuring the purity of deposited semiconductor layers during CVD processes is becoming a major concern as process pressures and deposition temperatures decrease and as the reactant gases are energized to increasingly higher levels.
Much attention has been given of late to low temperature CVD processes. Indeed, a number of polymer and low melting point metal films require a low processing temperature to avoid defects such as dislocations, interstitial species, stacking faults and the like. For example, final passivating films (such as silicon nitride) over aluminum metallization must be formed at temperatures less than 400.degree. C. The need for ever lower temperatures has been instrumental in fueling the development of Plasma Enhanced Chemical Vapor Deposition (PECVD) devices, in which the creation of a plasma in a gas mixture produces chemically reactive species at low temperatures. In PECVD devices, RF frequency glow discharges creates high-energy electrons that dissociate and ionize gaseous molecules, to generate chemically reactive radicals and ions. Kinetic energies for the electrons in PECVD chambers generally range in the 10.sup.2 to 10.sup.4 eV range.
Plasma-Enhanced CVD, however, increase contamination problems, as the chamber walls are subject to high-energy ion bombardment. To prevent the deposited films from being contaminated with such impurities, a number of contaminant decreasing measures have been implemented. Examples of such measures include careful cleaning of the interior of the process chamber, careful handling of materials, and strict control of the purity of the chamber components. Another contaminant decreasing measure that has been adopted is a process that is alternatively referred to as chamber seasoning, conditioning or pre-coating. These terms refer to coating some portion of the interior surfaces of the process chamber with undoped silicate glass, or USG. As shown in FIG. 1, a typical wafer processing cycle may include a reactor dry clean step, followed by a USG pre-coat step and a wafer deposition step. The pre-coat step shown in FIG. 1 coats interior surfaces of the processing chamber with a USG layer that is believed to act as a barrier to underlying impurities, preventing the impurities from out-diffusing from the interior surfaces of the chamber. After pre-coating, as shown in FIG. 1, the wafers are processed and the requisite films deposited. After a predetermined interval or after the processing of a predetermined number of wafers, the process is repeated, whereupon the reactor is once again dry cleaned. Periodic Maintenance (PM) of the process chamber is also generally required. Typically, such Periodic Maintenance includes a wet cleaning step using, for example, isopropyl alcohol or de-ionized water. Other cleaning steps may include the introduction of a fluorine containing gas into the process chamber, such as NF.sub.3. Nevertheless, microcontamination levels from particles and ion contaminants, despite these precautions, have typically remained in the 1.times.10.sup.10 ions/cm.sup.2 range, even when a USG pre-coat step is carried out after the in-situ dry cleaning step.
Newer techniques for chemical vapor deposition have been developed. For example, High Density Plasma Chemical Vapor Deposition, or HDPCVD, was developed to fulfill the requirements of shrinking chip size. HDP-CVD has the ability to deposit oxides into small gaps while avoiding the formation of voids by combining low pressures, an inductive coupled plasma CVD and a plasma sputtering process. For this reason, HDP-CVD is also called a simultaneous deposition and etch technology. In HDP-CVD, a source plasma that is generated by inductive coupling RF power produces ions and contributes to deposition, while a bias RF power enhances ion sputtering at the surface of the wafers and also at the processing chamber surfaces. The high density and low pressure inherent in HDP-CVD devices, however, increases sputtering on the process chamber interior surfaces. It has been found that conventional techniques for controlling the contaminant levels have proven to be ineffective in reducing contaminants to acceptable levels in high-density plasma environments. Specifically, undoped silicate glass, while decreasing contaminant levels somewhat, has proven ineffective in bringing contaminant levels in HDPCVD reactors down to PECVD levels. Moreover, even extreme care in handling process chamber components and the use of very high purity reactor parts in combination with a long USG pre-coating step has not reduced contaminants to acceptable levels, particularly for applications that are highly sensitive to contaminants. Indeed, certain processes such as Shallow Trench Isolation or STI, while benefiting from deposition in an HDPCVD environment, are extremely sensitive to impurities that degrade their performance.
What is needed, therefore, is a method of further reducing impurity levels in thin films to be deposited in chemical vapor deposition devices. Indeed, there has been a long felt need for a method to reduce contaminant levels in high plasma density, low pressure and temperature chemical vapor deposition devices to levels that are acceptable to the most demanding applications.